The invention relates to apparatus for purifying viscous materials, in particular to rid a plastic melt of solid contaminants.
The European patent document A1 615 825 discloses apparatus allowing to separate solid contaminants, for instance metal particles or the like, from a viscous thermoplastic melt. The known apparatus comprises a tubular-cylindrical sifting device fitted with a plurality of narrow holes in its casing. The sifting device is seated in a support pipe containing a plurality of circumferentially arrayed axial ducts to evacuate the purified material and merging at the inside surface of the support pipe into circumferential channels open toward the apertures of the sifting device. The circumferential channels may be formed either into the sifting device or into the support pipe. To carry out purification, the plastic melt is forced by an extruder into the cavity of the filter and is evacuated in purified form from the outside of the filter through the ducts of the support pipe. A rotating scraper shaft coaxially mounted in the filter scrapes the filtered contaminants from the filter. To prevent clogging the filter apertures of the filter, the hole diameter may not drop below a minimum size, entailing the drawback of degrading the purity of the purified plastic.
It is further known from the German patent document C1 35 35 491 and the Swiss patent document 469,547 to press the plastic melt to be purified by an extruder through a narrow-mesh wire cloth filter band resting at the exit side of the purified plastic melt on a support fitted with a plurality of comparatively large passages. The sifting band retains the solid contaminants to be separated and is moved by a hydraulic conveyor or by hydrostatic forces exerted by the melt along the supporting surface of the support in order to move the solid contaminants out of the flow path of the plastic. In the known apparatus the support surface is planar and the sifting band also is moved along a plane. Overall, however, the area of the of the sifting belt cross the flow path of the plastic is comparatively small and the purification output of the apparatus will not meet high requirements.
The molten plastic to be purified is highly viscous and accordingly requires high feed pressure which the narrow-mesh, preferably metal-woven filter band, cannot withstand per se at substantially large surface. The ratio of support surfaces where the filter can be supported on one hand to aperture cross-sections of the passages issuing into the support surface on the other hand must be selected in relation to the particular strength of the filter band.
Conventional supports are fitted with a plurality of boreholes arranged in a grid and passing many times through the lamellar support. However supports of this design cannot meet the higher requirements of filter performance because excessively small boreholes tend to clog and excessively wide boreholes cannot adequately brace the filter band. Moreover the borehole size affects the mechanical strength of the support and limits its load-bearing area to comparatively small values if an excessively bulky and hence heavy design of the purifying apparatus must be avoided.
The Japanese patent document A 61-175 016 (Patent abstracts of Japan M-548, 24 Dec. 1986, vol. 10/#385) and also U.S. Pat. No. 4,080,297 disclose that the filter band is guided over part of the circumference of a circular-cylindrical, hollow support drum fitted at its circumference with a plurality of radial slits. The plastic melt to be purified is fed radially from the outside to the filter band and is evacuated in purified form from the inside of the support drum. The wrap angle of the filter belt may amount to 180xc2x0 and more, and accordingly a comparatively large effective filter surface is possible. Similar filter systems are described in U.S. Pat. Nos. 3,645,399 and 3,856,674. However as regards purifying apparatus with a rotating support drum, not only is it necessary to provide sufficient sealing of the intake and outlet ducts of the filter band relative to the housing on account of the high feed pressure of the plastic melt, but furthermore the support drum must be sealed in the zone of these ducts. Wedge-shaped cutters are integrated into the housing in the vicinities of the feed side and the evacuation side of the filter band and penetrate at the feed side or at the discharge side between the filter band and the circumferential surface of the support drum. Because such cutters must be of a minimum thickness on account of the high prevailing pressures, the filter band is strongly deformed especially when passing from the support drum to the evacuation-side cutter, and thereby this filter band is susceptible to damage.
Compared with rotating support drums, the filter belt used with stationary supports is exposed to very high motional drag already stopping the filter band from being advanced at a comparatively small effective filtering area as long as it will be exposed to the high pressure of the plastic melt. The high pressure of the plastic melt forces the filter belt into the plurality of passages of the stationary support and the filter then snags on the support. In order to nevertheless move the filter band, the pressure of the plastic melt is lowered in conventional purification apparatus on the feed side. Illustratively it is known from the German patent document C1 35 35 491 to enlarge the volume of the feed chamber to further lower the pressure. The European patent document A2 0,275,462 discloses alternatingly feeding the plastic melt to be purified through a three-way valve to two purifying apparatus. The particular purifying apparatus not in the purifying mode therefore is unpressurized and its filter band thus can be moved. In spite of the decrease in feed pressure of the plastic melt, the motional drag of the filter belt remains relatively high as before, and on that account the effective filtering area must be kept comparatively small when using a stationary support.
The object of the invention is to create apparatus to purify viscous materials, in particular to rid a plastic melt of solid contaminants, such apparatus on one hand being compact and on the other hand allowing purifying the material to high purity and with comparatively high purification outputs.
The apparatus of the invention for purifying a viscous material, in particular to rid a plastic melt of solid contaminants, comprises:
a housing having a feed duct for the material to be purified and an evacuation duct for the purified material,
a filter band/belt, hereafter generally called merely xe2x80x9cfilter beltxe2x80x9d, displaceable in its longitudinal direction, to separate the solid contaminants and passing through the material""s flow path in sealed manner between the feed duct and the evacuation duct,
a stationary support structure in the flow path on the side of the filter belt facing the evacuation duct and comprising a support surface supporting the filter belt of which the contour follows the filter belt over a curved segment of the support surface wrapping the filter belt over a wrap angle large than 90xc2x0, said segment being substantially in the form of a cylindrical surface, in particular a circular cylindrical surface and being substantially planar in the support surfaces which adjoin tangentially and tightly the curved support surface, fitted with a plurality of mutually parallel channels running next to each other in the direction of displacement of the filter belt and open toward the support surface at least within the curved support surface, and fitted with a plurality of passage ducts starting from the channels to move the purified material to the evacuation duct,
a conveyor to advance the filter belt in continuous or step-wise manner.
The purification apparatus is very compact because of the curvature of the support structure, even when the effective area of the filter belt is comparatively large. Moreover such a support structure better withstands the high feed pressure of the plastic melt, which may reach 300 bars or more. Also the dead volume of the plastic melt in such a design is comparatively minor, thereby facilitating caloric control of the purification process.
Because the planar support surfaces adjoin the curved one in tangential and flush manner, that is continuously and without a jump, the filter beltxe2x80x94contrary to the conventional case of purification apparatus with rotating support drumsxe2x80x94is not mechanically deformed when entering or leaving the curved support surface, this feature being especially advantageous when the filter belt must be cleaned and reused. Because at least the curved support surface is fitted with channels running in the direction of advance, the motional drag on the filter belt can be kept comparatively low. Even if the filter belt were to deform into the channels on account of the pressure of the plastic melt, it can nevertheless be moved along the channels since the passage ducts issue into the channels, not directly into the support surface.
In a preferred embodiment, the planar support surfaces are fitted at least in a small zone adjoining the curved support surface with a plurality of mutually parallel channels running in the direction of advance of the filter belt and open toward the support surface, and being the starting points of the passages moving the purified material to the evacuation duct. Such a design makes possible extraordinary large effective filter surfaces, in particular when the wrap angle by which the filter belt wraps the curved support surface is about 180xc2x0 or even larger.
A stable design which nevertheless offers adequate support even for large, effective filter surfaces, is achieved if the support structure comprises a plurality of connection ducts forming the passages and running transversely to the channels and each time intersecting the cross-sectional contour of several channels in order to form passage holes. Such a support structure is can be manufactured at low cost and in particular as a solid, integral block. Moreover the connecting ducts can be connected in such a support structure to an evacuation-duct manifold issuing substantially centrally to them. Not only does this design reduce the dead volume of plastic melt within the housing, but also this objective can be attained at low manufacturing cost while assuring the mechanical strength of the support structure. Conceivably the connection ducts might issue laterally of the support structure into a slotted, radially flaring part of the evacuation duct. However in such a case when considering the desirability of minimal dead volume of plastic melt, the first duct sections of the connection ducts, which preferably run parallel within the support structure to the cylinder generatix of the curved support surface, appropriately are each connected by particular, associated, radially running second duct sections to the manifold duct.
During purification, the filter belt is loaded with the solid contaminants in the region of the channels. Even though with respect to minimal motional drag the channels ought to be free of transverse jumps, a preferred embodiment nevertheless requires that the channels within a first partial zone of the support surface shall terminate within this surface and in that as seen in the direction of advance, in a second partial zone of the support surface, channels therein shall start within the support surface, the channels of the second partial zone being staggered behind and relative to the channels of the first partial zone. Such a channel configuration allows using the entire filter-belt surface for purification and as a result the filter-belt service life is substantially lengthened.
Between the beginnings of the channels of the second partial zone of the support surface, that is between the ribs remaining between these channels, the support structures may be fitted with slanted surfaces to minimize the motional drag of the filter belt.
Seen in the direction of advance of the filter belt, the length of the first and second partial zone of the support surface is selected in such manner that the filter belt shall be loaded with contaminants as uniformly as possible. Appropriately the ends of the channels of the first partial zone of the support surface and the beginnings of the channels of the second partial zone are situated approximately at the middle of the support surface as seen in the direction of advance of the filter belt.
The feed channel may issue into the housing while being coaxial with the feed duct. As a result the integration of the purification apparatus into a more complex plastic processing plant or the like will be facilitated. On the other, in an especially simple design, the feed duct issues into the housing of the feed chamber by means of a duct section which runs approximately radially to the support surface. In this manner the dead volume of material to be purified within the housing can be further reduced while nevertheless assuring uniform loading of the filter belt. Preferably, as seen in circumferential direction of the support surface, the duct section issues approximately centrally into the feed chamber.
In an especial compact design of the purification apparatus, the filter belt will wrapxe2x80x94as already mentionedxe2x80x94the support by at least 180xc2x0, and is guided by means of at least approximately mutually parallel planar belt segments within the housing in an essentially flat belt feed chamber toward the support structure and further is moved away from the support structure in an essentially flat belt evacuation chamber. The belt feeder chamber and the belt evacuation chamber are configured to be mutually parallel to contribute to easier temperature control of the housing.
When purifying thermoplastics, the material is kept at the melting point in the region of the support structure, and depending upon the material, the housing is illustratively kept at a temperature from 180 to 330xc2x0 C., through also high or lower, using temperature control. On the other hand, in the region of the belt feeder chamber and/or of the belt evacuation chamber, the housing is cooled in order to seal at least the evacuation site of the filter belt by solidifying material. In order to attain this objective with a compact housing, the belt feeder chamber and/or the belt evacuation chamber is partitioned by a heat insulation means transverse to the plane of segments of the filter belt guided therein into a temperature-controlled zone near the support structure and a cooling zone far from the support structure.
In a preferred embodiment, the belt feeder chamber and/or the belt evacuation chamber will widen at least in a partial zone in the direction of advance of the filter belt transversely to the plane of this band. The widened belt evacuation chamber is approximately situated in the cooling zone and allows evacuating the contaminant-loaded filter belt, whereas the this widening of the belt feeder chamber will narrow again near the support drum and then facilitate the temperature control of the material to be purified.
In particular the belt evacuation chamber, though also the belt feeder chamber, can be adequately sealed by a xe2x80x9cplugxe2x80x9d of solidified plastic melt in the manner discussed above. However it was found that temperature control is particularly problematic at the side of belt evacuation because there the filter belt is loaded with contaminants which strongly affect the solidification of the plastic melt. The time interval required to change the overall state of the plastic melt may rise considerably and thereby adversely affect the apparatus"" purification performance. Drawbacks of this sort can be avoided, as called for in a preferred embodiment, by placing, at least on the side of the belt evacuation chamber, a sealing strip displaceably guided transversely to the plane of the filter belt and made to rest, in sealing manner, between a sealing position while tightly resting on the filter belt and a release position away from the filter belt. In order that the sealing strip displaceable relative to the housing may be guided in sealing manner despite the high pressure of the plastic melt, the sealing strip is made to rest flat by its side surface away from the flow path of the material to be purified transversely to the direction of advance of the filter belt and across the full width of the filter belt against a sealing surface of the housing. In such a design the pressure of the molten plastic simultaneously will produce the compression of the mutually associated and preferably even ground surfaces. In this design the sealing strip runs transversely to the filter belt and projects beyond its longitudinal edges and is also sealed in that zone by its side surfaces relative to the housing sealing surface. Accordingly when the sealing strip rests against the filter belt, it may enclose same completely and assure annular sealing. Appropriately the sealing strip is metallic and fitted on its side facing the filter belt with an elongated seal made of elastomers or another elastic material. Even though such a seal is especially suitable to seal the belt evacuation chamber of the above discussed purification apparatus, such sealing also can be advantageously used for other purification apparatus of the species.
The drive means moving the sealing strip between its two positions is appropriately a hydraulic actuator or the like. Such or another drive means is appropriately mounted outside the flow path of the plastic, that is, in an unpressurized region. Preferably the actuation forces are transmitted by at least one cylindrical dowel guided in displaceably manner in a dowel borehole toward the sealing strip. Appropriately the dowel is a loose pin to circumvent jamming. Preferably at least two dowels are mounted on each side of the sealing strip and a distance apart from each other, the dowel apertures running in the direction of motion of the sealing strip, the drive means actuating the dowels acting on oblique thrust surfaces.
The movement of the filter belt can be implemented by hydrostatic-pressure differences along the flow path of the plastic melt. Specifically regarding embodiments wherein the pressure of the plastic melt is lowered during conveyance to lower the motional drag on the filter belt, preferably a conveyor moving the filter belt is provided. The conveyor may be in the form of a pair of compressing rollers. However preferably it shall be hydraulically displaceable and hydraulically actuated clamping tongs which in their reciprocating motion grips and clamps the filter belt. However the clamping tongs also may be in the form of carriages hydraulically displaced along a linear guide of the filter belt.
Conventional purification apparatus of the kind being discussed herein guide a filter band from a supply coil. Depending on the degree of soiling of the material to be purified and the degree of purity of the purified material, conduction of filter band may be comparatively large. To minimize filter-band consumption, a preferred embodiment of the invention designs it as an endless belt which in its direction of advance will cross a cleaning station offset from the support structure. The cleaning of the filter belt takes place in continuous or stepwise manner depending on the advance of the filter belt and not only saves filter-band material, but also allows lowering the pressure of the material on the feed side of the material to be purified. In the state of the art, high material pressures even at near full clogging of the filter belt caused by the contaminants have been allowed to achieve economical filter-band operation. If, on the other hand, as proposed herein, the filter belt is cleaned while the material is being purified, the degree of clogging of the material can be reduced by appropriately selecting the speed of advance of the filter belt and thereby the pressure of the material, without incurring time losses for belt changes or higher consumption of filter belts.
The above discussed concept of an endless filter belt with associated cleaning station not only applicable to purifying of the above discussed kind, but also to conventional cleaning equipment of independent inventive significance.
As regards a preferred embodiment, the filter belt is guided in endless form around the curves support surface of the support structure and around at least one rotatably supported reversing wheel. The reversing wheel may be a drive reversing wheel powered by the conveyor means. The reversing drive may constitute the sole drive for the filter belt, optionally all reversing wheels being driven.
The concept of cleaning the filter belt immediately upon material purification in a cleaning station to allow re-using the filter belt, is applicable not only to endless filter belts but also to apparatus where the filter band comes from a supply coil. The contaminated filter belt/band coming from the material purification apparatus is restored by this cleaning to such flexibility that it can be wound and used again. In a preferred embodiment the support structure is mounted in the path of advance of the filter band between a supply coil feeding the filter band and a winding system for the filter band, a cleaning station offset from the support structure being mounted in the path of advance and crossing the filter band. Again as was the case above for the material purification apparatus fitted with an endless belt, the cleaning station is applicable not only in material purification apparatus of the above kind, but also to conventional cleaning stations of independent inventive significance.
In both the above-discussed variants, the cleaning station preferably is in multistages to implement a cleaning effect matched to cleaning state. In a preferred embodiment the cleaning station comprises sequentially, as seen in the direction of advance of the filter belt, a scraper device and then at least one cleaning device. In this respect it was found advantageous for purifying thermoplastics to fit the scraper system with heaters preheating the filter belt and the at least one subsequent cleaning station with heaters raising the temperature of the at least one subsequent cleaning station beyond the preheating level. The cleaning station following the scraper system may be a fluidized bed cleaning station wherein a flow of particles, for instance sand, is applied to the filter belt. The fluidized bed cleaning station appropriately is operated at a temperature substantially higher than the thermoplastic melting point, for instance being at least 400xc2x0 C., whereas the scraper system operates in the range of the material""s softening point, for instance roughly 200xc2x0 C. These temperatures however are material-dependent.
To carry out the final cleaning of the filter belt, one of the cleaning devices may be in the former of a blower illustratively blowing hot air through the filter belt.
As a rule the housing must be disassembled to install the filter belt into it. In a preferred embodiment much facilitating inserting or changing the filter belt, the housing comes in section and optionally comprises a multi-part, removable wall segment covering the edge of the filter belt over its full length inside the housing. By removing this cover-like wall segment, the edge zone of the filter belt will be uncovered over its full length inside the housing and as a result the filter belt can be removed transversely to its direction of advance from the housing, or be inserted in it. In this design the feed duct and the evacuation duct appropriately issue into the residual housing, And consequently these duct hookups need not be loosened to change a filter belt.